The continuing growth in radio telecommunications is perpetuating the need to increase the capacity of cellular systems. The limited availability of the frequency spectrum mandates that future systems use efficient methods to increase network capacity and to adapt to various traffic situations. Although the introduction of digital systems is increasing system capacity, these improvements alone are not sufficient. To further increase system capacity the size of the cells in metropolitan areas must be decreased to meet the growing demands.
FIG. 4 illustrates two cell clusters A and B forming part of a cellular mobile radio telephone system in a manner well known in the art. Typically, all the frequency in a system are used in each cell cluster. Within the cell cluster, the frequencies are allocated to different cells to achieve the greatest uniform distance, known as their frequency reuse distance, between cells in different clusters using the same frequency. In FIG. 4, cells A.sub.1 and B.sub.1 both use a common frequency as do cells A.sub.2 and B.sub.2, cells A.sub.3 and B.sub.3, etc. The radio channels in cells A.sub.1 and B.sub.1 using the same frequency are referred to as co-channels because they share the same frequency. Although some interference will occur between co-channels, the level of such interference in an arrangement such as that of FIG. 4 is normally acceptable. The cell plan of FIG. 4 therefore allows for a relatively simple frequency allocation and should provide, for some systems, an acceptable low level of co-channel interference.
It is also well known in the art for radio base stations located near the center of each cell (or near the center of three adjacent "sector cells") to provide radio coverage throughout the area of the cell. The cell plan of FIG. 4 assumes a relatively uniform distribution of mobile radio telephone users throughout the area of a cell. To handle relatively dense concentrations of mobile users, a preferable arrangement is to establish localized microcells. Microcells allow additional channels to be physically located in close proximity to where they are actually needed, boosting cell capacity while maintaining low levels of interference. Microcells may cover thoroughfares such as crossroads or streets; a series of microcells may provide coverage of major traffic arteries such as highways. Microcells may also cover large buildings and shopping malls.
Microcells are the wave of the future. With the introduction of microcells, radio network planning increases in complexity. The planning process is dependent on the structure of the microcells, i.e. the size of streets and buildings. Microcells suffer from a series of problems including an increased sensitivity to traffic variations, interference, and difficulty in anticipating traffic intensities. Even if a fixed system could be successfully planned, a change in system parameters such as adding new bases to sustain increased traffic demand would require replanning the system. For these reasons the introduction of microcells will require a system in which channel assignment is adaptive both to traffic conditions and to interference conditions.
In order to solve the aforementioned difficulties which accompany the implementation of a microcellular system, it is necessary to develop a dynamic planning process to account for changing system requirements. A method to satisfy this need involves dynamic channel allocation which can also be used in macro cell environments. Several methods have been used to develop a dynamic planning process.
One approach is to use a centralized decision for the channel assignment for each new call as disclosed in an article entitled "A High Capacity Assignment Method for Cellular Mobile Telephone Systems" by Nettieton and Schloemer in 39th IEEE Veh. Tech. Conf., San Francisco, May 1989, pages 359-367. Reportedly, this system provides significant capacity improvement. However, it is important to note that the capacity improvement estimates were based on a simulated system which contained only 19 cells. Since call initiations were logged over all cells and not just for the inner portion of the simulated system, it is possible that some of the improvement can be attributed to not having a fully developed interference environment.
Nevertheless, centralized channel assignment systems perform better than systems where the channel assignment decision is made locally. In local channel assignment schemes, the effect of interference from a new connection on established connections is not known in advance. Moreover, centralized methods more effectively reuse the channels geographically.
Systems using centralized methods are not without major drawbacks. For example, increased signaling in the network and extensive calculations are required.
In light of the drawbacks associated with centralized methods, there has been considerable testing of methods where the channel assignment decision is made locally, only considering the new connection to be established. However, when testing these methods it is of great importance to consider the interference for all connections at times not related to the time a channel is assigned to a new call or to a handover, when a call is switched from one channel to another during the call. This is necessary although a channel is not allocated unless it has a quality exceeding the lower quality limit of the system, because establishing a new connection may cause non-negligible interference on other calls. Therefore, when comparing different methods, it is important to not only consider the blocking of a new call, but also the probability that a connection has a quality below an acceptable level.
The article "Strategies For Handover and Dynamic Channel Allocation In Microcellular Mobile Radio Systems" by Beck and Panzer in the 39th IEEE Veh, Tech, Conf., San Francisco, May 1989, at pages 668-672 and the article "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communication Systems" by Furuya et al. in DMR II, Stockholm 1986 at pages 311-315, survey different methods of locally allocating channels. When a new connection is established there is a set of channels which fulfills the minimum quality criteria. The difference between these methods is how the channel is selected.
The lowest probability for blocking arises when the channel which has the lowest carder to interference (C/I) ratio is selected, but the probability of interference using this method is too high. In another method, the channel with the highest C/I level is selected. Although this method does not use the frequency spectrum as efficiently as a fixed planned system, the probability of interference is very low. In a system with a small number of channels, the trunking gain (i.e., where two channels take more than twice the load of one channel with the same probability for blocking a new call) is much larger than the loss in frequency spectrum efficiency.
A third method called regulated DCA (dynamic channel allocation) employs an interference margin on the C/I level and selects the channel with the lowest C/I level above the minimum C/I for the system plus the margin. By providing different margins (targets), this method has some flexibility to change the density in the reuse pattern of the channels and to make tradeoffs between blocking and handover failure.
In another method called a segregation method, each base station maintains a priority list of all channels, ordered according to how often the channels have been successfully used. Thus, a base station will preferentially use channels with which it has a past history of successful uses. When a new call is established the channels are scanned beginning at the top of the list and the first free channel which fulfills the predefined quality criteria is allocated to the new call. Consequently, neighboring bases often will find these channels to be of low quality when testing them and accordingly assign them a low priority. After an initial time period, a self planned system will evolve where the selection of channels which may interfere with calls in neighboring cells or which may suffer substantial interference from a neighboring base will be less frequent. The segregation method can be combined with regulated DCA whereby a channel is not selected unless it has a C/I level above the target level. Accordingly, the channel with the highest priority and a C/I level above the target C/I is selected and not the channel with C/I closest above the target.
The present invention provides a method of channel allocation for use, for example in the American Digital Cellular system (ADC), with little if any change in the existing TIA standard IS-54. This standard allows for the mobile station to measure signal strengths of the frequencies specified by a base station connected to the mobile station, and signal quality of the down link of the channel in use. The quality measurement is used to check that a channel selected by the base is acceptable for the mobile after the connection is established. The signal strength and quality measurements act as a guide to determine whether handover is necessary. The base station, however, selects the traffic channel for handover and the timing of the handover.
IS-54 also imposes some restrictions on the method to be used. For example, the standard does not allow for mobile measurements of free channels prior to a new call set up. However, the present approach can be implemented in a system where both the mobile and base station can measure on free channels prior to allocation. Also, to facilitate the introduction of diversity in the mobiles, base stations will transmit continuously on a frequency once a call is established on any time slot of that frequency. As a result, it is difficult for the mobile to measure the interference of the free channels on the same frequency since the dominant signal strength will arise from the corresponding base.
To solve these problems, the channel that is selected should, with a high probability, be of acceptable quality on the down link. In accordance with this requirement, a DCA segregation scheme was derived. The method does not restrict the number of channels used in each cell. Instead, the method chooses a channel, among the channels having the requisite quality, to be allocated as a traffic channel. Further, the method can handle inhomogeneous traffic loads equally as well as the aforementioned methods.
In addition to not having to plan the frequencies in the system, several advantages over fixed channel allocation are realized by the dynamic channel allocation approach. The probability of blocking a call at call set up is reduced. Likewise, the probability of losing a call prior to normal termination and the probability of interference, particularly co-channel interference, are reduced resulting in enlarged system capacity. However, base station costs are increased since extra transceivers need to be installed in the base stations and since the transceivers and combiners (without having multi-carrier power amplifiers) must be tunable. Nevertheless, this extra cost must be compared to the cost of increasing system capacity by installing additional bases. As cell sizes get smaller and smaller, the necessity to use dynamic channel allocation increases. Further, a capacity increase is realized even with only a moderate increase in the number of transceivers.